Combustion of solid fuel in a fluidized bed of solid material such as bubbling (BFB) and circulating fluidized beds (CFB), is known to be advantageous in many respects. Due to the low furnace temperature and the use of hot circulating solids, many beneficial features, such as low NOx and SOx emissions, fuel flexibility, and capability of using low grade fuel, are achieved in a CFB process. Capturing of many substances present in or originating from the combustion reactions may be readily accomplished by appropriately selecting the bed material that is used. For example, sulfur emissions may be considerably reduced by using materials that react with sulfur to facilitate sulfur capturing.
CFB combustion is well known as an air combustion process, an example thereof being discussed in a Chinese patent publication. Patent CN 1566749 A, which also shows a method of implementing a high pressure fluidization blast using steam, instead of air, in normal operation of a boiler. The fluidization steam, source pressure is greater than 60 kPa, and the superheating degree is more than 30° C.
New regulations and other demands limiting the gas emissions related to the so-called greenhouse effect have encouraged development of new technologies aimed towards decreasing carbon dioxide emissions from power stations using fossil carbonaceous fuels. While, in conventional firing, the oxygen required for burning the fuel is fed in the form of air, in oxycombustion, the air is replaced with a mixture of substantially pure oxygen and recycled flue gas, which may be called an oxidant.
Oxycombustion is based on combusting carbonaceous fuel with substantially pure oxygen, typically, of at least 95% purity, so as to have carbon dioxide and water as the main components of the exhaust gas discharged from the boiler. Thereby, the carbon dioxide can be captured relatively easily, without having to separate it from a gas stream having nitrogen as its main component, as when combusting the fuel with air.
Once of the mechanisms of sulfur capture in the furnace, when limestone is used, is as follows. The limestone calcines in the furnace to form calcium oxide:CaCO3→CaCO+CO2.Calcium oxide reacts with SO2 to form calcium sulfate by a reaction:CaO+SO2+½O2→CaSO4.Another known mechanism is direct sulfatizing with a reaction:CaCO3+½O2+SO2→CaSO4+CO2.CaSO4 being a solid material, may be removed from the gas by separation. The course of the reactions is naturally dependent on the prevailing temperature and, particularly, on the partial pressure of the CO2.
CaSO4, CaCO3, as well as CaO, are efficiently mixed into the bed material of the CFB boiler and, thus, they are also present in the external circulation of solid material of the CFB boiler. The formed calcium sulfate can, thus, be removed from the furnace together with the ashes. Therefore, a combustion process using a circulating fluidized bed boiler does not necessarily need additional sulfur reducing equipment in the exhaust gas channel, or the efficiency of such equipment can be relatively low.
In order to maintain the optimum temperature for low emissions and high combustion efficiency within the furnace of a CFB boiler, a sufficient heat transfer surface must be provided to remove heat from the combusting products. The heat transfer surface can be provided by arranging a fluidized bed heat exchanger (FBHE) in the external circulation of solid material, which cools the externally circulated solids before they are returned back to the furnace. This is also the case with oxycombustion CFB boilers.
U.S. Pat. No. 6.505,567 discloses a circulating fluidized bed steam generator and a method of operating the same. A fluidized bed heat exchanger is also shown to be coupled to the external circulation of solid material. The method includes the step of introducing a substantially pure oxygen feed stream into the circulating fluidized bed steam generator and the step of combusting a fuel in the presence of the substantially pure oxygen feed stream, to produce a flue gas having carbon dioxide and water vapor as its two largest constituent elements by volume. The method includes the step of separating the flue gas into an end product portion and a recycling portion and directing the recycling portion of the flue gas to the circulating fluidized bed steam generator to contribute to the combustion process therein. The recycling portion of the flue gas is also used for fluidizing gas in the fluidized bed heat exchanger.
It has been recently discovered that in an oxycombustion CFB boiler, there may occur disadvantageous behavior in the solids handling system due to the presence of CaO, when the fluidization of solid material is accomplished, making use of a recycled portion of the flue gas containing a high concentration of CO2. This is due to the tact that CaO has a tendency to recarbonate back to CaCO3 in certain circumstances.